Pneumatic tires are sometimes intended for heavy duty use such as example for hauling and for traveling over uneven ground when carrying heavy loads which might be referred to as being off-the-road service. Exemplary of such tires are medium truck tires such as, for example, dump trucks and waste haul trucks.
However, sometimes such pneumatic tires are also intended for such heavy duty use to be driven on diverse surfaces including a combination of both roadways and also off-the-road conditions which present more severe use conditions for the tires.
A combination of such intended diverse mixed service use conditions presents significant challenges for rubber compositions for treads of such tires. A balanced combination of improving abrasion resistance properties for the tread rubber while maintaining other physical properties is sometimes difficult to obtain. Here, a methodology of achieving such result is to be evaluated, particularly for a rubber composition for a tire tread intended for such heavy duty service.
For heavy duty tire tread work, the principal (primary) elastomer used for the tread rubber composition is often natural rubber (natural cis 1,4-polyisoprene rubber). The tread rubber may contain a minor amount of cis 1,4-polybutadiene rubber to promote abrasion resistance and thereby resistance to tread wear to promote a greater vehicular driving distance relative to depth of tread worn away.
Such natural rubber-rich treads are in contrast to passenger tire treads for lighter intended duty which are expected to be driven over dedicated roadways while experiencing lower workloads and which may typically be comprised primarily of synthetic rubber such as, for example, styrene/butadiene rubber with a minor amount of cis 1,4-polybutadiene rubber which may also contain a minimal amount of natural rubber, if any.
For example, rubber compositions for natural rubber-based treads of such heavy duty tires for such mixed service may be desired which promote a relatively low hysteresis property for the tread rubber for promoting reduced internal heat buildup in the tread during tire service with a corresponding beneficial increased heat durability of the tread and predictive beneficial reduction in tire rolling resistance for better fuel economy for an associated vehicle.
For the beneficial promotion of reduction in hysteresis property for the natural rubber-based tread rubber composition, it may be thought of to employ filler reinforcement for the rubber composed of precipitated silica reinforcement with a significantly reduced content of rubber reinforcing carbon black.
However, such reduction in rubber reinforcing carbon black content for the tread rubber might be expected to promote a reduction in abrasion resistance and therefore promote a reduction in resistance to tread wear.
A challenge is therefore presented of promoting an increase in abrasion resistance without sacrificing various other physical properties of the tread rubber composition.
For such challenge it is proposed to evaluate an inclusion of a fatty acid amide in combination with precipitated silica in a form of pre-hydrophobated precipitated silica in a natural rubber based rubber composition.
Such fatty acid amides may be represented by a general formula (I):
where R1 is comprised of an alkyl hydrocarbon radical containing from 12 to 36 carbon atoms, an alkenyl hydrocarbon radical containing from 12 to 36 carbon atoms or alkadiene hydrocarbon radical containing from 12 to 36 carbon atoms.
In one embodiment, the fatty acid amide may be an amide of saturated or unsaturated monovalent amines or of saturated or unsaturated polyvalent amines.
For example, the fatty acid amide may be comprised of an amide of at least one of caprylamine, lauralamine, palmitylamine, stearylamine, oleylamine, myristylamine, methylenediamine, ethylenediamine, hexamethylenediamine and ammonia.
In one embodiment, the fatty acid amide may be comprised of at least one of caprylamide, lauralamide, palmitylamide, stearylamide (stearamide), oleylamide and myristylamide.
In one embodiment, the rubber composition may contain from 1 to about 10, alternately from about 1 to about 25, parts by weight of the fatty acid amide per 100 parts by weight of the rubber (phr).
For ease of handling, the fatty acid amide may be provided as being deposited on a suitable carrier. Representative examples of such carriers may be, for example, silica, carbon black, alumina, kieselguhr, silica gel and calcium silicate.
In practice, such pre-hydrophobated precipitated silica may be a precipitated silica pretreated with at least one of bis(3-triethoxysilylpropyl) polysulfide with an average of from about 2 to about 4 connecting sulfur groups in its polysulfidic bridge or pretreated with an alkoxyorganomercaptosilane optionally together with at least one of alkylsilane and fatty acid.
Representative examples of pre-hydrophobated precipitated silica are, and not intended to be limiting, U.S. Pat. Nos. 6,573,324 and 7,368,587.
In the description of this invention, terms such as “compounded rubber”, “rubber compound” and “compound”, if used herein, refer to rubber compositions containing of at least one elastomer blended with various ingredients, including curatives such as sulfur and cure accelerators. The terms “elastomer” and “rubber” may be used herein interchangeably unless otherwise indicated. It is believed that such terms are well known to those having skill in such art.